1. Technical Field
The present invention relates generally to integrated circuits and in particular to improved protection devices for integrated circuits. Still more particularly, the present invention relates to an improved device and method for manufacturing electrostatic discharge and overvoltage protection devices for integrated circuits.
2. Description of the Related Art
Electrostatic discharge (ESD) phenomena commonly result from pulses of high voltage, (typically, several kilovolts), moderate current (a few amperes) and short duration (on the order of 100 nanoseconds) generated by a variety of sources, such as human bodies, electric fields generated by machinery, and the like.
ESD effects are a common problem in integrated circuits (IC) electronics and are particularly troublesome in complimentary metal-oxide-semiconductor (CMOS) devices, which have particularly thin gate oxides and very short channel devices. Such structures can typically withstand only a few tens of volts. An ESD pulse conducted through a CMOS IC can induce oxide rupture and device or interconnect burnout. An ESD pulse can also induce a "latch-up" condition in thick field devices.
ESD problems can be abated somewhat by including appropriate anti-static protection on circuit boards on which the ICs are installed. The ICs, however, still are susceptible to ESD before and during the circuit board fabrication process. A general personal awareness of ESD problems and countermeasures, such as electrically grounded bracelets and the like reduce risks in this phase of the IC lifespan. It is, however, at best, a partial solution. Therefore, it is desirable to include ESD protection circuitry within the IC itself.
Overvoltage pulses caused by reflection of voltage pulses result in voltages much higher than the operating voltage of the IC. Such a situation occurs with SCSI devices that are connected to a SCSI bus containing other active SCSI devices. In such situations, overvoltage pulses return from the bus to the ICs in the SCSI device. These pulses can damage the gates or devices in the IC. Typically, diodes are added across the pads of the chip to protect the IC from overvoltage pulses. Traditional diode structures, however, typically have capacitances that are higher than desired for SCSI devices when added externally to the IC. In particular, small computer system interface (SCSI) devices can only tolerate a 1 pF loading capacitance. Line bounce may cause an increase in voltage up to two times the original voltage.
Therefore, it would be advantageous to have an improved method and device that can be used to provide both ESD and overvoltage protection for ICs.